A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to accurately control the position of the patterning device relative to the wafer or substrate, a lithographic apparatus is often provided with one or more positioning devices for positioning an object table e.g. holding a patterning device or a substrate. Such positioning devices can e.g. comprise one or more linear actuators such as Lorentz actuators for accurate (short stroke) positioning of an object table or support. In such case, a first member of such Lorentz actuator (comprising one or more permanent magnets and optionally a magnetic yoke) is mounted to the object table or support that needs positioning whereas a second member of the actuator (comprising a coil for generating a magnetic flux and optionally a magnetic yoke) is mounted to a long-stroke mover. Such an actuator is configured to generate a force between the first and second member that is solely dependent on the current supplied to the coil and as such is independent of the position of the first member relative to the second member. Using a Lorentz actuator, such a force characteristic can be approximated within a specified operating range of the actuator.
In a lithographic apparatus, both accurate positioning and throughput (e.g. expressed as the number of wafers that can be processed per hour) can be considered equally important. In order to obtain a high throughput, powerful actuators and motors are required enabling high accelerations and decelerations of the object table thereby reducing any idle time between consecutive exposures. In order to meet these requirements, it is observed that known Lorentz actuators are found to have a comparatively small force density or force vs. dissipation, compared to other types of actuators such as variable reluctance actuators. It has been observed that a first member of such Lorentz actuator (comprising one or more permanent magnets and optionally a magnetic yoke) has a comparatively large volume and mass resulting in a comparatively high total mass to be displaced by the linear actuators of the positioning device. Further, such actuators are found to have a comparatively high electric power requirement (and consequently a comparatively high dissipation) which may adversely affect amplifier demands. Compared to Lorentz actuators, a variable reluctance actuator would enable a improved force density while at the same time reducing (moving) mass of the actuator and the dissipation level. Known variable reluctance actuators however suffer from the drawback that an accurate force control is rendered difficult because the actuator force is strongly dependent on the relative position of the magnetic members of a variable reluctance actuator. Further, is has been observed that known variable reluctance actuators may suffer from having a comparatively high cross-talk, i.e. in addition to generating a force in a desired direction, variable reluctance actuators as known may suffer from generating disturbance forces and/or torques which render it more difficult to obtain an accurate positioning of e.g. an object table using such actuators. Such cross-talk in general depending on the relative position between a first and second member of the actuator. As such, using known variable reluctance actuators, it may be difficult to predict the actuators response when a certain magnetizing current is applied. As such, compensating for such generated disturbance forces and/or torques referred to as cross-talk may be difficult as well thereby adversely affecting the positioning accuracy that can be obtained.